The present invention relates to methods to produce refined microstructures in gamma titanium aluminide alloys.
It is known that as aluminum is added to titanium metal in greater and greater proportions, the crystal form of the resulting titanium aluminum composition changes. Small percentages of aluminum go into solid solution in titanium and the crystal form remains that of alpha titanium. At higher concentrations of aluminum, including about 25 to 35 atomic %, an intermetallic compound, Ti.sub.3 Al, is formed. The Ti.sub.3 Al has an ordered hexagonal crystal form called alpha-2. At still higher concentrations of aluminum, including about 49 to 60 atomic %, another intermetallic compound, TiAl, is formed having an ordered, face-centered tetragonal crystal form called gamma.
The alloy of titanium and aluminum having a gamma crystal form, and a stoichiometric ratio of approximately one, is an intermetallic compound having a high modulus, a low density, a high thermal conductivity, favorable oxidation resistance, and good creep resistance. It has been shown that TiAl has the best modulus of any of the titanium-based alloys. Not only is the TiAl modulus higher at higher temperature, but the rate of decrease of the modulus with temperature increase is lower for TiAl than for the other titanium alloys. Moreover, the TiAl retains a useful modulus at temperatures above those at which the other titanium alloys become useless. Alloys which are based on the TiAl intermetallic compound, so-called "gamma" TiAl alloys, have attractive attributes for use where high modulus is required at high temperatures and where good environmental protection is also required.
One of the characteristics of TiAl alloys which limits their actual application to such uses is a brittleness which is found to occur at room temperature (RT). Also, the strength of the TiAl alloys at RT needs improvement before the TiAl intermetallic compound can be exploited in many structural component applications.
It is known that the addition of a small quantity of boron to gamma titanium aluminide improves the RT ductility of the intermetallic compound. U.S. Pat. No. 5,080,860, issued Jan. 14, 1992, to Shyh-Chin Huang, discloses that the addition of 0.5 to 2 atomic percent boron to gamma titanium aluminide alloys improves the castability and the RT ductility of the intermetallic compound. However, the addition of a lesser quantity of boron, i.e., 0.1 to 0.2 atomic percent boron, to gamma titanium aluminide alloys did not provide any significant improvement in the values of the tensile and ductility properties. U.S. Pat. No. 5,205,875, issued Apr. 27, 1993, to Shyh-Chin Huang, discloses that cast and forge processing together with the addition of 0.1 to 0.2 atomic percent boron to gamma titanium aluminide alloys does yield a ductility advantage.
The properties of alloys based on gamma titanium aluminide depend strongly on microstructure, as well as composition. It has been shown that the most desirable microstructures are based on the lamellar structure which consists of alternating layers of TiAl plates and Ti.sub.3 Al plates. Such structures are commonly called "fully-lamellar microstructure" (FL) and can be produced: (1) in cast alloys during cooling after solidification or (2) in wrought processed alloys during cooling after holding in the so-called high-temperature alpha field. In either case, however, the resulting FL microstructures consist of large grain sizes which are typically larger than 400 .mu.m. Investigations have shown that these large grained FL materials exhibit poor tensile properties, although their resistance to fracture initiation/growth and high-temperature creep are quite remarkable. This unbalanced relationship has attracted a great deal of attention for the last several years. A few methods have been proposed or introduced to improve the balance of properties by reducing the FL grain sizes.
In casting alloys, TiB.sub.2 additions were found to reduce the cast lamellar grain size drastically when the amounts exceed a certain level, that is, about 0.8 vol % TiB.sub.2 (about 0.9 at % boron). "XD" alloys such as Ti-(45 or 47)Al-2Mn-2Nb-0.8 vol % TiB.sub.2 are typical examples in that the as-cast material contains randomly oriented fully-lamellar grains having a uniform grain size of about 150 .mu.m and "jagged" grain boundaries.
For wrought-processed alloys, grain size control has been attempted through several different routes. Two thermomechanical processing routes, (1) high-temperature extrusion and (2) forging followed by heat treatment, were disclosed in U.S. Pat. No. 5,226,985, issued Jul. 13, 1993 to Y-W. Kim and D. Dimiduk. The first method is referred to as a thermomechanical process (TMP) and comprises shaping the article by extrusion or hot die forging, rolling or swaging, followed by a stabilization aging treatment. In this process, shaping is carried out at a temperature in the approximate range of 0.degree. to 20.degree. C. below the alpha-transus temperature (T.sub..alpha.) of the alloy. The alpha-transus temperature generally ranges from about 1300.degree. to about 1400.degree. C., depending on the alloy composition. T.sub..alpha., decreases with decreasing Al. The alpha-transus temperature has also been shown to decrease with many interstitial (e.g., O and C) and substitutional (e.g., Cr, Mn, Ta and W) alloying elements. T.sub..alpha. can be determined relatively routinely by standard isothermal heat treatments and metallography, or by Differential Thermal Analysis (DTA), provided the material is homogeneous.
The aging temperature can range between 750.degree. and 1100.degree. C., depending on the specific use temperature contemplated, for at least one hour and up to 300 hours. The TMP method provides a product with a fine lamellar microstructure.
The second method is referred to as a thermomechanical treatment (TMT), which comprises hot working at temperatures well below the alpha-transus (T.sub..alpha.) with subsequent heat treatment near the alpha-transus followed by a stabilization aging treatment. Where shaping is by extrusion, extrusion is carried out at a temperature in the approximate range of T.sub.e -130.degree. C. to T.sub..alpha. -20.degree. C. Where shaping is by hot die forging, rolling or swaging, such shaping is carried out at a temperature in the approximate range of T.sub.e -130.degree. C. to T.sub..alpha. -20.degree. C., at a reduction of at least 50% and a rate of about 5-20 mm/min. Where shaping is by isothermal forging, such shaping is carried out at a temperature in the approximate range of T.sub.e -130.degree. C. to T.sub.e +100.degree. C., at a reduction of at least 60% and a rate of about 2-7 mm/min. After hot working, the article is heat treated at a temperature in the approximate range of T.sub..alpha. -5.degree. C. to T.sub..alpha. +20.degree. C. for about 15 to 120 minutes. Following such heat treatment, the article is cooled and given an aging treatment. The TMT method provides a product having randomly oriented lamellar microstructures which appear similar to those in cast XD material.
A modification to the TMT forging+heat treating method is disclosed in SIR H1659, issued Jul. 1, 1997, to S. Semiatin, D. Lee and D. Dimiduk. This method provides an alternative for controlling grain size which is more suited to production heat treating, especially for thick cross-sections.
TMP processing was further developed to include wider ranges of temperatures and detailed extrusion conditions and parameters in U.S. Pat. No. 5,417,781, issued May 23, 1995, to P. McQuay, D. Dimiduk and Y-W. Kim. McQuay et al disclose four methods: The first of these methods comprises the steps of: (a) heat treating an alloy billet or preform at a temperature in the approximate range of T.sub..alpha. to T.sub..alpha. +100.degree..degree. C. for about 0.5 to 8 hours, (b) shaping the billet at a temperature between T.sub..alpha. -30.degree. C. and T.sub..alpha. to produce a shaped article, and (c) aging the thus-shaped article at a temperature between about 750.degree. and 1050.degree. C. for about 2 to 24 hours. The second method comprises (a) rapidly preheating an alloy preform to a temperature in the approximate range of T.sub..alpha. to T.sub..alpha. +100.degree. C., (b) shaping the billet at a temperature between T.sub..alpha. and T.sub..alpha. +100.degree. C. to produce a shaped article, and (c) aging the thus-shaped article at a temperature between about 750.degree. and 1050.degree. C. for about 2 to 24 hours. The preform is held at the preheat temperature for 0.1 to 2 hours, just long enough to bring the preform uniformly to the shaping temperature. The third method comprises the steps of: (a) heat treating an alloy billet or preform at a temperature in the approximate range of T.sub..alpha. to T.sub..alpha. +100.degree. C. for about 0.5 to 8 hours, (b) rapidly heating the preform to shaping temperature, if the shaping temperature is greater than the heat treatment temperature, (c) shaping the preform at a temperature between T.sub..alpha. and T.sub..alpha. +100.degree. C. to produce a shaped article, and (d) aging the thus-shaped article at a temperature between about 750.degree. and 1050.degree. C. for about 2 to 24 hours. The fourth method comprises the steps of: (a) heat treating an alloy billet or preform at a temperature in the approximate range of T.sub..alpha. -40.degree. C. to T.sub..alpha. for about 0.1 to 2 hours, (b) rapidly preheating the preform to shaping temperature, (c) shaping the preform at a temperature between T.sub..alpha. and T.sub..alpha. +100.degree. C. to produce an shaped article, and (d) aging the thus-shaped article at a temperature between about 750.degree. and 1050.degree. C. for about 2 to 24 hours.
These methods generate unique lamellar microstructures consisting of randomly oriented lamellar colonies, with serrated grain boundaries. Gamma titanium aluminide alloys with such structure have the requisite balance of properties for moderate and high temperature aerospace applications: high specific strength, stiffness, fracture resistance and creep resistance in the temperature range of room temperature to about 950.degree. C.
Another method disclosed in U.S. Pat. No. 5,558,729, issued Sep. 24, 1996, to Y-W. Kim and D. Dimiduk, describes an alloy modification to expand and lower the high-temperature two-phase (alpha and beta) field. In this case, the annealing treatment is done in the two-phase field, instead of the single-phase alpha field, which results in reduced grain size due to the competition between the two phases. The resulting, fine lamellar microstructures are called refined fully-lamellar (RFL) microstructures and their grain sizes range relatively widely from 100 to 500 .mu.m.
While the methods described above for refining lamellar grain sizes are valuable contributions to the art, each of the methods has drawbacks, in that the processing windows are relatively narrow, and are not particularly tolerant of derivations from the prescribed windows. In general, they are perceived as being insufficiently "robust" for large scale commercial production of gamma alloy components.
Accordingly, it is an object of the present invention to provide improved methods for producing articles of gamma titanium aluminide alloys.
Other objects and advantages of the invention will be apparent to those skilled in the art.